To allow fingerprints to be identified, images of the fingerprints are analyzed, this being done by extracting characteristic features, particularly endings and branchings of the ridges on the skin, and comparing them with reference features that are held in store.
To enable the characteristic features to be extracted, it is necessary for the images to be pre-processed and the details produced by the pre-processing to be pre-processed and stored. A step that is frequently taken as part of the pre-processing is so-called skeletonization, in which the ridges, which are a plurality of picture elements (pixels) wide, are reduced to lines only one pixel wide. Before this is done, the brightness levels are turned into binary values. To store a line of this kind, all the raster dots that go to make up the line therefore have to be stored in the memory together with their co-ordinates. Given the large number of lines that there are, this takes up a considerable amount of space in the memory.
It is therefore an object of the invention to encode the lines in a manner that reduces the data without there being any loss of the information required for structural analysis.